difference in pressure drop between stationary and transient flow

[mike.h]

Hello,

i am investigating the Influence of Inertia in a turbulent free jet. Therefore my strategy was to deduct the pressure drop of a stationary solution from the pressure drop of a transient flow (e.g. decrasing the Flow rate at Inlet).

The problem that i have right now is that the pressure at the Inlet will drop apx. 10% as soon as i switch to transient mode, even if the flow rate is held constant for a certain period of time. In other words: there is a difference between the prediction of the pressure drop at a choke point depending on if i simulate in stationary mode or transient mode (in transient mode i chose to set the Boundary conditions to the same constant value). Can someone explain this circumstance please?

p.s. I am using the k-e model with advection scheme 2

kind regards,

mike

[OmkarJ]

When you say the pressure drops as soon as you switch from steady to transient, do you mean during the iterations, or after you have obtained a converged solution using steady state solution? 

Typically for any sudden change, or even stopping and starting in steady simulation, I have seen sudden drop/increase of average values (plots). This perhaps is due to the fact that momentum terms are recalculated, which should affect the pressure values in the domain as well, because of pressure-velocity copupling in seggregated solver. If you wait for a while, you'll see the plots behaving normally.

OJ

[mike.h]

Hi OJ and thanks for your answer.

I let the steady state converge (minimal fluctuations of pressure at the Inlet). Then i switch to transient mode with a (constant) transient condition at the inlet (velocity= 1.69702 m/s) 

The transient solution has converged after apx. 500 timesteps with a difference of apx. 11percent within the Inlet pressure. I have attached the pressure plot of a monitor point at the Inlet which resembles the bulk result very well.

[OmkarJ]

Just a quick question, if steady solution exists, why do you bother to run it as transient? One would run the transient solution only if the flow properties depend on time and the transient term in NS equations need solution, which doesn't seem the case in your situation.

Agreed, the problem that is steady, will yield (nearly) same solution when run as steady state or transient. But you have to be sure that transient simulation is adequately converged.  Given your timestep of 1e-7 s compared to the flow rate, the change per iteration imight be so small that you are interpreting this as convergence. If you increase the timestep, try in multiple of 10, do you still see the pressure flat/increase/divergence?

OJ

[mike.h]

The reason why i need the steady state solution is mentioned in my first post. The flow i am simulating is transient with a changing flow rate. I stumbled upon this problem while the analysis of the pressure drop due to inertia.

The seemingly low flow rate will result in very high maximum velocities (~160 to 200 m/s) in the small orifice that will cause the free jet flow. If i set the time step smaller than 5e-6s, the pressure at the inlet is not converging (will oscillate between 8e7 Pa and vapor pressure).

I will let it run for more timesteps with the current settings and see if i notice an upward direction of the pressure curve.

thanks for your help